Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), astigmatism vision errors, and defects in near range vision usually associated with aging (presbyopia).
It is believed that presbyopia occurs as a person ages when the lens of eye begins to crystallize and lose its elasticity, eventually resulting in the eye losing the ability to focus at near distances, such as the normal reading distance, and in some cases at intermediate distances. Presbyopic persons (presbyopes) complain of difficulty performing close tasks. To compensate for presbyopia, ophthalmic lenses are required that are more positively powered or less negatively powered than the distance correction. Some presbyopic persons have both near vision and far vision defects, requiring bifocal or multifocal lenses, instead of single vision lenses, to properly correct their vision.
Large populations of presbyopes also have an astigmatism vision error. Astigmatism is optical power meridian-dependent refractive error in an eye. This is usually due to one or more refractive surfaces, most commonly the anterior cornea, having a toroidal shape. It may also be due to one or more surfaces being transversely displaced or tilted. Astigmatism is usually regular, which means that the principal (maximum and minimum power) meridians are perpendicular to each other. People with astigmatism have blurred vision at all distances, although this may be worse at distance or near, depending on the type of astigmatism. These people may complain of sore eyes and headaches associated with demanding visual tasks. Astigmatism can be corrected with an astigmatic ophthalmic lens, which usually has one spherical surface and one toroidal (cylindrical) surface.
However, current contact lenses are unable to correct both astigmatism and presbyopia. Therefore, there is a need for a contact lens having a cylindrical optical surface (or power) to correct astigmatism vision errors and a multifocal power to compensate for presbyopia.
An object of the invention is to provide a contact lens having a cylindrical optical surface (or power) to correct astigmatism vision errors and a multifocal power to compensate for presbyopia.
Another object of the invention is to provide a method for producing a contact lens having a cylindrical optical surface (or power) to correct astigmatism vision errors and a multifocal power to compensate for presbyopia.
A further object of the invention is provide a family of contact lenses having a series of different cylindrical power corrections and a multifocal power to compensate for presbyopia.